1. Field of the Invention
The present invention relates to a film covered battery which contains a battery element sealed by a casing made of a film.
2. Description of the Related Art
In recent years, batteries for use by portable devices as power sources are strictly required to be increasingly lighter in weight and smaller in thickness. Such tendency also affects a battery casing so that a conventional metal can, which is limited in a reduction in weight and thickness, has been gradually replaced by a metal foil film or by a laminate film made of a metal foil and a heat-sealable resin film laminated thereon, which is used as a casing because it can be further reduced in weight and thickness and can be made in a free shape, as opposed to the metal pack.
A representative example of laminate film suitably used for a battery casing may be a three-layer laminate film which is comprised of an aluminum foil, which is a metal thin film, a heat-sealable resin film laminated on one side of the aluminum foil to function as a heat seal film, and a protective film laminated on the other side of the aluminum foil.
Generally, in a film covered battery which employs a casing made of a laminate film, a battery element including a positive electrode, a negative electrode, an electrolyte, and the like are covered with the casing with the heat-sealable resin film being located inside, and the casing is thermally fused around the battery element to seal the battery element. The heat-sealable resin film is made, for example, by a polyethylene film or a polypropylene film, while the protection film is made, for example, of a nylon film or a polyethylene terephthalate film.
Lead terminals are connected to the positive electrode and negative electrode, respectively, for leading out the electrodes to the outside of the casing, and the lead terminals are extended from the casing. The lead terminals are connected to the battery element by ultrasonic welding or the like prior to the sealing of the battery element. Generally, the battery element is sealed using two casing materials by sandwiching the battery element between the two casing materials, and thermally fusing the casing materials around the overall peripheries thereof. In this event, three sides of the casing materials may be first thermally fused into a bag-like shape, followed by thermal fusing of the remaining sides of the respective casing materials in a vacuum atmosphere, permitting the casing materials to come into close contact with the battery element.
When the battery element has a certain degree of thickness, one casing material may be previously formed in a collared cup shape such that the battery element can be readily received thereby. Then, the cup-shaped casing material is put over the battery element to cover it. This has been generally done in practice.
JP-2001-126678-A discloses a laminate polymer electrolyte battery which has a battery element comprised of multiple positive electrodes and negative electrodes. Each of the electrodes has a protruding tab coated with an electrode material. The positive and negative electrodes are laminated together with separators interposed between each positive electrode and each negative electrode. The separators are impregnated with an electrolyte solution. The tabs of the positive electrodes and the tabs of negative electrodes are collectively ultrasonic welded, respectively, to form charge collectors for the positive electrode and negative electrode, respectively, in the battery element. The charge collectors are connected to lead terminals of the positive electrode and negative electrode, respectively. Then, the battery element having the lead terminals connected thereto is placed on a flat enclosure material, the other cup-shaped enclosure material is placed over the flat enclosure material with the battery element included therein, and the two enclosure materials are thermally fused along their peripheries to create the laminate polymer electrolyte battery. In the laminate polymer electrolyte battery thus fabricated, the charge collectors are formed by applying pressure to the tabs of the positive and negative electrodes from above using an ultrasonic welding head, and the battery element is sealed using the two enclosure materials as mentioned above. With the use of the two enclosure materials in the shapes mentioned above, the lead terminals are led out near the bottom of the battery element.
A battery having a battery element comprised of alternately laminated positive electrodes and negative electrodes is referred to as a “laminate battery.” There is another type of battery referred to as a “wound-type battery.” This type of battery has a battery element comprised of alternately laminated positive electrodes and negative electrodes. A strip-shaped positive electrode and negative electrode are laid one on the other with an intervening separator sheet therebetween, wound around, and compressed into a flat shape to form the alternately laminated positive electrodes and negative electrodes.
The battery elements also include one having a charge accumulating function such as a capacitor, other than chemical batteries such as a lithium battery, a nickel hydrogen battery, and the like. By now, the capacitor is also fabricated using a casing made of a laminate film.
Further, the laminate film has been used for casings for use with batteries directed to large equipment such as a hybrid car, which are required to provide high power and large capacity, because a battery covered with a casing made of the laminate film can have increased electrode areas in spite of its thin and light-weight feature, as compared with batteries using a metal can.
The conventional laminate polymer electrolyte battery mentioned above, however, has a problem in that the lead terminals can extend obliquely from the enclosure materials. This phenomenon will be described below with reference to FIGS. 1 and 2.
As illustrated in FIG. 1, enclosure material 102a, which coveres battery element 105, is formed into a cup shape through a deep drawing process. For this reason, a recess formed in enclosure material 102a for receiving battery material 105 has a diverging side surface, causing the formation of a space between battery element 105 and enclosure material 102a. Also, particularly in the laminate polymer electrolyte battery, charge collector 109 is formed for collectively connecting a plurality of positive electrodes and a plurality of negative electrodes to associated lead terminals 103, respectively. Charge collector 109, however, protrudes from part of battery element 109, and has a smaller thickness than battery element 105, so that it is a general tendency from a viewpoint of facilitating the deep draw process of enclosure material 102a that the recess of enclosure material 102a is formed in a shape which is designed out of consideration of charge collector 109, and generally in a substantially rectangular shape including a portion in which charge collector 109 is located.
As a result, before battery material 105 is sealed as illustrated in FIG. 1, a large space is formed between enclosure material 102a and battery element 105 particularly in a region in which charge collector 109 is formed. As described above, battery element 105 is carried on flat enclosure material 102b, and the space is formed due to the recess of enclosure material 102a which covers battery element 105.
If battery material 109 is sealed in the state illustrated in FIG. 1, charge collector 109 and lead terminal 103 are pulled up, causing enclosure materials 102a, 102b to cave in and deform into unintended shapes, as well as charge collector 109 and lead terminal 103 to bow. Bowed charge collector 109 and lead terminal 103 will result in lead terminal 103 extending obliquely with respect to the thickness direction of battery element 105. Disadvantageously, obliquely extending lead terminal 103 would be an impediment when the laminate film electrolyte battery is mounted on an associated device.
Lead terminal 103 bows larger as battery element 105 has a larger thickness, and particularly, the thickness of battery element 105 exceeding 3 mm makes lead terminal 103 more susceptible to bowing.